Current techniques for Ultra-Violet Lithography (UVL) are well known and established within the semiconductor fabrication industry. For example, ultra-violet (UV) light having a wavelength of 193 nm is used in the process of photo-etching a semi-conductor microstructure using combinations of positive and negative photo-resists (PR), UV sources, and lensing apparatus. One example of a lens used in this process is a fused silica glass (SiO2); however, technology in lens manufacture is investigating the use of lenses made of other materials, such as calcium fluoride. The continued use of technology employing fused silica lens becomes increasingly difficult due to absorption characteristics of the lens at wavelengths much below 193 nm. For example, the extreme ultra-violet (EUV) systems of 153 nm and less suffer from near total absorption by SiO2 lenses. As a result, there is continued interest in the development of a calcium fluoride lens.
The smallest structure that can be created by a “dry” lithography UV system is about 100 nm and is referred to as the resolution of the system. The resolution of a lithography system is limited by the nature of light passing from one medium to another, the resolving power of the lens used, and Rayleigh's Criterion. Rayleigh's Criterion is a function of the wavelength of a light source, the refractive index “n” of a lens, the physical diameter of the lens, and the angle separating the maximum and minimum points of diffraction of the light source through the lens.
One way of increasing the resolving power of a lens is to increase the diameter of the lens while maintaining a constant focal length, which in turn increases the numerical aperture of the lens (a term commonly used in photography to express capturing the detail of a subject by increasing the amount of light incident on a lens). However, according to Snell's Law of Reflection, the angle that a light ray can intercept the boundary layer between two refractory mediums before there is total reflection of the light ray within the medium with a higher refractive index is limited. Therefore, the practical size of lens that may be used in the dry process is restricted.
Because of the technical difficulties encountered developing EUV systems, “Immersion Lithography” is being investigated as an alternative to conventional dry lithography processes. Immersion Lithography is a “wet” process that uses water as an intermediary refractive layer between the lens and the wafer substrate being exposed to an UV light source. One advantage of IL is that water has a higher refractive index (e.g., about 1.4 at 29° C.) than air (e.g., about 1.0). The index of refraction of pure water is significantly closer to the index of refraction for a silica lens than air. Therefore, bending is reduced, a larger lens may be used, and the numerical aperture and the resolving power of the system are increased. As a result, the resolving power of the lens/water system is substantially greater than the lens/air system
The refractory properties of water in all of its physical states have been thoroughly investigated. The index of refraction of water is based on the physical system under which it exists and the wavelengths of light to which it is being exposed. The refractive index for water can be determined a number of ways, although direct observation for a fixed set of conditions is the most reliable. Good approximate values can be calculated for a wide spectrum of wavelengths using a derived formula first presented in a paper by P. Schiebener et al. and later modified by Levelt Sengers et al. The formula is as follows:(n2−1)/(n2+2)*(1/ρ)=ao+a1ρ+a2T+a3λ2T+a4/λ2a5/(λ2−λ2uv)+a6/(λ2−λ2IR)+a7ρ2
a0=0.244257733 a4=1.58920570×10−3 λUV=0.229202
a1=9.74634476×10−3 a5=2.45934259×10−3 λIR=5.432937
a2=−3.73234996×10−3 a6=0.900704920 T=normalized
(K)
a3=2.68678472×10−4 a7=−1.66626219×10−2 λ=normalized
ref-ρ*=1000 kg m−3 ref-T*=273.15° K. ρ=normalized
ref-ρ*=0.589 um
Values close to points of discontinuity may be approximated through extrapolation and checked through observation. The above formula uses reference values to derive dimensionless constants in order to take into account the variables of density, temperature, and wavelength. The formula itself was numerically derived using observed values and a curve-fitting algorithm. Extrapolated results have been demonstrated to give good approximate values. Other formulas to derive the refractive index of water using observed dielectric values may be found.
Two processes being investigated using water as an intermediary layer are briefly described in an IEEE article, “Chip Makings Wet New World” by Linda Geppart, Spectrum-IEEE Press, May 2004 and are briefly described here. One method uses total immersion lithography in which the lensing system and wafers are completely immersed in water during the lithography process. However, full immersion of the system provides slower processing times due to the inertial forces of the water. For example, the inertial forces affect the velocity of moving the wafer, fluctuations in optical properties due to currents in the water and temperature gradients, and the inability to properly control the creation of air bubbles either through cavitations of the moving mechanisms or air bubbles trapped on the wafer's photo-resist during immersion (which affect the number of defects created on the substrate during processing). In addition, particle and chemical contaminants are introduced in the liquid medium with one possible particle source being the photo-resist layer itself.
A second method being developed uses a water injection system whereby a layer of water is injected via nozzles located around the lens onto a wafer substrate. After the wafer has been exposed to the UV light source, the water is “sucked up” by the injector system. However, the water injection system also produces air bubbles within the water layer that are introduced by the injectors in addition to air bubbles trapped on the photo-resist as the water is sprayed over the surface and contaminants are introduced within the medium due to the injection of water directly onto the photo-resist resulting in photo-resist surface ablation.